There exist boom energy regenerating devices for work machines including a work unit with a boom and adapted to expand/constrict a boom cylinder by switching a control valve to drive the boom with a view to achieving both an increase in the amount of energy regeneration and the improvement of operability at the same time at a high level without causing an abrupt change in operability. Patent Document 1, for example, discloses such a boom energy regenerating device for work machines, the device including a branching section that branches a hydraulic fluid line for a return fluid from the boom cylinder into two or more lines during downward movement of the boom, a recovery circuit that guides one of the branched fluid lines to a tank via regenerating means, and a flow control circuit that guides the other of the branched fluid lines to the tank via flow control means. The recovery circuit for guiding the fluid to the tank via the regenerating means is disposed outside the control valve.